The present invention relates to viscometers of the type for measuring liquid viscosities based upon rotational deflections of a suspended bob.
There have been many prior designs of viscometers that utilize the torsion wire principle. Early designs made use of a relatively heavy bob suspended on a thin wire. The bob was suspended in the liquid to be measured and a container holding the liquid was rotated. The twisting force imparted to the bob via the liquid was measured by a device attached to the wire. In these designs, the weight of the bob was relied on to keep the wire centered. This, in turn, limits their use to xe2x80x98thin liquidsxe2x80x99 and thin wires.
Other designs have used thicker wires or rods but these designs lack the sensitivity required for the fluids they desire to study. Designs such as U.S. Pat. No. 4,299,118 employed a bob mounted on a rod with the rod attached to a fine torsion spring. The rod is centered by ball bearings which are also used to center a rotatable sleeve, concentric with and centered about, the bob. This arrangement allows any fluid container to be used but inaccuracies occur because of mechanical friction transmitted from the driven sleeve to the bob through the bearings.
More recently, designs have appeared such as my earlier U.S. Pat. No. 5,763,766 where multiple fixation points are used to tension and center thicker wires. However, because the drive to the rotating container is from beneath the bob, special containers have to be used to control the temperature of the sample. The design was modified in U.S. Pat. No. 6,070,457 to replace the lower mechanical fixing point for the wire with a concentric repelling magnet arrangement and, in fact, bears a striking similarity to the apparatus described in U.S. Pat. No. 4,045,999, In both of these examples, special containers are still required for controlling the sample temperature.
The entire disclosures of U.S. Pat. Nos. 4,045,999, 5,763,766, and 6,070,457 are incorporated herein by reference.
It will be deduced by those skilled in the art that, although angular torsion or deflection is measured by an electronic non-contact sensor and transducer, the path traveled by the sensor is, in fact, an arc and therefore the response of the transducer cannot be linear due to the curvature of the arc. As noted by Simon and Heflinger, stable levitation (or repulsion) of one magnet by another is prohibited by Earnshaw""s Theorem and this effect is seen as a tilt of tube 66 in U.S. Pat. No. 6,070,457. The combination of these two effects means that the instrument described in U.S. Pat. No. 6,070,457 requires a sophisticated calibration routing to linearize the output from the transducer and, as a consequence, the instrument must be manually xe2x80x98zeroedxe2x80x99 before each sample measurement.
Prior viscometers of the types discussed above are illustrated in U.S. Pat. Nos. 1,192,861; 1,236,706; 1,281,042; 2,203,132; 2,303,162; 2,398,574; 2,957,339; 3,435,666; 4,045,999; 4,242,086; 4,299,118; 5,763,766; and 6,070,457 and in European Application Nos. 007427; 311301; 384792; and 449586 and PCT Application Nos. WO 91/06364; WO 91/14168; WO 92/10763; and WO 92/06365.
It is clear from the teachings of these patents that the authors consider the centralizing of the bob to be of importance and none have considered the location of the torque sensing device. In many of the above examples mirrors or sensors are simply attached to the torsion wire and it is clear that any bowing of the wire or off center orbiting will translate into a movement of the sensor that is not due to the fluid under investigation.
The apparatus of the present invention represents an improvement over the viscometer designs of the prior art and, in particular, the Palmer U.S. Pat. No. 4,045,999 and Robinson U.S. Pat. No. 6,070,457. The apparatus of the present invention uses any container for the fluid under test, rather than the specially designed temperature control unit and rotating cup of Robinson.
The apparatus of the present invention preferably uses a linear distance measurement device which is an optical encoder system of high accuracy. The use of this device eliminates calibration routines and any subjective intervention by an operator. In one embodiment of the invention, a freely swinging wire can be used. The encoder scale is attached to the end of the wire and a housing is attached to some point on the wire. The encoder read head (preferably wireless) is mounted on the housing opposite the scale. The two components are coupled together with a magnetic bearing. By fixing the encoder scale and the read head in this manner, even if the wire swings, the two will move as one and thus record no relative movement between each other.
In a first aspect, the present invention provides a viscometer for measuring liquid viscosities based upon rotational deflections of a suspended bob, wherein the improvement comprises: a deflection indicator; a deflection reader located at a spaced relative position with respect to the deflection indicator effective for reading the deflection indicator; and a rotating element mounted for rotation in unison with the bob. The rotating element includes one of the deflection indicator and the deflection reader. The improvement further comprises a magnetic bearing assembly which retains at least the rotating element in a manner effective to allow the rotating element to rotate in unison with the bob while preventing any substantial change in the spaced relative position of the deflection reader with respect to the deflection indicator. As used herein and in the claims, the phrase xe2x80x9cany substantial changexe2x80x9d in the spaced relative position of the deflection reader with respect to the deflection indicator refers to any change in relative position exceeding the critical position of tolerances between the indicator and the reader.
In a first embodiment of this aspect of the invention, the improvement further comprises a suspended housing which holds the other of the deflection indicator and the deflection reader. The rotating element is preferably positioned on the suspended housing. Additionally, the magnetic bearing assembly preferably comprises a first magnet included in the rotating element and a second magnet positioned in the suspended housing adjacent to and spaced apart from the first magnet. The magnetic bearing assembly more preferably comprises a third magnet positioned in the suspended housing above and spaced apart from the first magnet with the second magnet being positioned below the first magnet. Further, the rotating member preferably does not contact the suspended housing.
The deflection indicator employed in the inventive apparatus is preferably a deflection scale and is most preferably an optical encoder scale. The deflection indicator is preferably included in the rotating element.
In a second embodiment of the first aspect of the invention, the improvement further comprises a suspended, rigid structure having the bob extending from a lower end thereof with the rotating element being retained on the rigid structure. This embodiment preferably further comprises a frame such that the magnetic bearing assembly comprises a first magnet retained on the rigid structure and a second magnet held in the frame at a position spaced above the first magnet. The magnetic bearing assembly also preferably comprises a third magnet held in the frame at a position spaced below the first magnet. The rigid structure is preferably suspended through the second and third magnets such that the second and third magnets surround but do not contact the rigid structure. The magnetic bearing assembly also preferably comprises a fourth magnet retained on the rigid structure below the rotating element with the first magnet being retained on the rigid structure above the rotating element and the third magnet being spaced below the fourth magnet. The magnetic bearing assembly most preferably further comprises a fifth magnet retained on the suspended, rigid structure at a position above and spaced apart from the second magnet.
The second embodiment can further comprise a flexible suspension element extending into the upper end of an interior passage provided in the rigid structure. The flexible suspension element can be, for example, a torsion wire. Alternatively, the second embodiment can further comprise at least one torsion spring connection between an outer portion of the rotating element and the frame.
In a second aspect, the present invention provides a viscometer for measuring liquid viscosities based upon rotation deflections of a suspended bob wherein the improvement comprises: a frame for suspending the bob; a sleeve rotatably positioned around the bob; and a sleeve holder having a lower portion from which the sleeve extends and having an upper portion rotatably retained in the frame such that the sleeve holder and the sleeve can be rotated by driving the sleeve holder at a location above the bob. This improvement preferably comprises a pulley secured on the sleeve holder above the sleeve for driving the sleeve holder. The improvement also preferably includes a bearing which rotatably retains the upper portion of the sleeve holder in the frame such that the pulley is positioned above the bearing.
Further objects, features, and advantages of the present invention will be apparent to those skilled in the art upon examining the accompanying drawings and upon reading the following description of the preferred embodiments.